<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <div>
        <button id="btn1">按钮1</button>
        <button id="btn2">按钮2</button>
        <button onclick="demo()">按钮3</button>
    </div>
</body>
<script>
    const btn1  = document.getElementById('btn1')
    btn1.addEventListener('click',function(){
        alert('按钮1')
    })

    const btn2  = document.getElementById('btn2')
    btn2.onclick = () =>{
        alert('按钮2')
    }

    function demo(){
        alert('按钮3')
    }
</script>
</html>